Conductivity measurement sensors are well known in the art and are used to measure the conductivity of a fluid, such as a liquid or a dispersion of solids suspended in a liquid. Conductivity sensors are often used to investigate the properties of electrolytes in solution, such as the degree of dissociation, the formation of chemical complexes, and the hydrolysis. The conductivity of a fluid may also be used to measure a wide variety of other parameters, such as the amount of contaminants in drinking water and a measure of chemical concentrations in industrial processes. Applications such as these involve the determination of conductivities in many physical environments.
The units of conductivity are Siemens/cm, which are identical to the older unit of mhos/cm. Conductivity measurements cover a wide range of solution conductivity from pure water at less than 1×10−7 S/cm to values in excess of 1 S/cm for concentrated solutions.
One conductivity measurement technique includes contacting a solution with electrically conducting electrodes. For example, one contacting conductivity measurement technique employs a sensor with two metal or graphite electrodes in contact with the electrolyte solution. An alternating current (AC) voltage is applied to the electrodes by the conductivity analyzer, and the resulting AC current that flows between the electrodes is used to determine the conductance. Contacting-type conductivity sensors generally employ two, or sometimes four, contacting electrodes, which physically contact the sample solution. In the case of four-electrode contacting sensors, the four-electrodes are exposed to the sample solution and a current is passed through one pair of electrodes. A voltage change between the other pair of electrodes is then measured. Based on the current and voltage, the conductivity of the liquid is calculated. Traditionally, contacting-type conductivity sensors, such as two or four-electrode sensors, are made by inserting conductive rods, (made of stainless steel, titanium, graphite, etc.) in a plastic tube, which rods are then sealed with epoxy along their length. The cross section of one end of the plastic tube is then used to expose the electrodes to the sample solution. FIG. 1A is a diagrammatic view of a four-electrode contacting-type conductivity sensor 10 in accordance with the prior art. Sensor 10 is coupled to a suitable conductivity analyzer 12. Distal end 14 of sensor 10 exposes distal ends 16 of conductive rods 18 to a sample solution disposed proximate distal end 14. FIG. 1B is a bottom plan view of sensor 10 illustrating distal ends 16 of rods 18.
Recently, contacting-type conductivity sensors, such as two and four-electrode conductivity sensors have been made by using semiconductor-like, planar manufacturing technologies. The electrodes are deposited on a passivated silicon wafer through suitable processing techniques, such as thin/thick film technology. Conductivity sensors manufactured in accordance with such semiconductor processing techniques can be mass-produced resulting in reduced size and cost of such sensors. However, the reduction in size of semiconductor-based conductivity sensors creates other manufacturing difficulties. Providing a semiconductor-based contacting-type conductivity sensor design that facilitated low-cost semiconductor-based manufacturing techniques would further benefit the art.